xDSL (DSL: Digital Subscriber Line) is a family of telecommunication services including ADSL (Asymmetric DSL), RADSL (Rate-Adaptive DSL), SDSL (Symmetric DSL) and VDSL (Very High-Speed DSL). DSL provides a digital connection for customers to other end users. By using digital signalling methods, both voice and data can be transmitted over telephone lines but at much higher speeds than those speeds that are possible with ordinary modems. Modems are communication devices which convert binary digital information into electrical signals for the transmission via telephone lines and which convert these electrical signals back into a binary form at the receiving end. A conversion into the electrical signal form is known as modulation, whereas the conversion back to the binary form is known as demodulation. Modems are used for data transmission from or to a data communication equipment such as a laptop or a personal computer via the telephone line.
In POTS (POTS: Plain Old Telephone System), the frequency range between 0 and 4000 Hz is known as the voice channel and the frequency range from 300 Hz to 3000 Hz is known as a telephone frequency band. The voice signals are transmitted within the telephone frequency band as in-band telephone signals, whereas the telephone signals transmitted between 3000 and 4000 Hz are known as out-of-band signals. The signals within the in-band transmit the voice information, whereas the out-of-band signals are used for signalling and control purposes. Any frequencies above 4000 Hz are filtered out so that they do not interfere with the voice signal.
ISDN (ISDN: Integrated Services Digital Network) where each telephone wire pair running to a home has the capability to carry not one but two simultaneous voice conversations, known as Bearer channels or B-channels. The voice calls are carried as 64 KPBS digital signals (128 KPBS together) along with a separate digital signalling channel, or D-channel. The D-channel can be used for packet data services when not needed for signalling, whereas the B-channels can be used for anything like voice, data and even video. ISDN transmits signals within a low frequency range reaching from 0 Hz to a cut-off frequency of about 130 kHz.
FIG. 1 shows an integrated telephone set having a modem according to the state of the art. The integrated telephone set comprises a conventional telephone set and an analog modem which are both connected to a switching device or multiplexer. The switching device is linked via a telephone line to the telephone network. The telephone wire is usually a single twisted pair wire made of copper. The analog modem has an interface for a connection to the data communication equipment such as a laptop or a personal computer. The integrated telephone set shown in FIG. 1 integrates the telephone functions and the data communication functions. However, in the integrated telephone set as shown in FIG. 1, the telephone services and the data communication services are both transmitted within the same frequency band. The telephone service is carried in-band together with the data communication service, and a fault in the data communication link will cause a failure of the telephone service.